In the mammalian reticulocyte, the synthesis of hemoglobin is a regulated process. This regulation is evidenced by a 1:1 molar ratio of the synthesis of alpha to beta peptide chains. Also, the rate of heme to globin synthesis is 1:1, i.e., one heme is synthesized for every alpha and beta peptide chain. The molecular mechanisms responsible for this regulated synthesis is the main concern of this research. Attention is being focused on three aspects of this problem: (I) To reinvestigate the disparity of the rates of alpha to beta drain syntheses at very early times in incubating rabbit reticulocytes. (II) To determine the relative (alpha vs beta) and absolute rates of chain- initiation in rabbit reticulocyte cells synthesizing hemoglobin. (III) The optimization of conditions for heme synthesis in isolated mitochondria from rabbit reticulocytes with a view toward reconstituting an in vitro system in which regulated synthesis occurs, and which will permit studying the assembly of the hemoglobin molecule.